1. Field of the Invention
The invention relates to a stepped optical fiber, which consists of multicomponent glass and comprises a core glass member and a cladding glass member completely surrounding the core glass member around its peripheral wall. It also relates to the use of this sort of stepped optical fiber.
2. Description of the Related Art
Generally a glass fiber or optical fiber for light transmission is made from a highly refractive core glass member and a cladding glass member surrounding it, which has a lower index of refraction than the core glass. A light transmitting glass body in fiber form, in which the index of refraction is constant across the cross-section of the core glass member, is called a stepped optical fiber. A glass fiber of this type transmits light, which is coupled into one end of the optical fiber, to the other end of the optical fiber. The light is completely reflected (total reflection) at the boundary surface between the core glass member and the cladding glass member.
The amount of light, which can be coupled into this sort of optical fiber and which can be transmitted by it, is proportional to the square of the numerical aperture (NA) of the fiber and the cross-sectional area of the fiber core. In order to transmit as much light as possible over short to medium distances (<100 m), these types of stepped fibers are frequently tightly packed fiber bundles, provided with a protective tube. Their ends are glued into a metal sleeve and their front faces are worked to form optically plane surfaces by grinding and polishing. The corresponding packaged optical fiber bundle is called a fiber optic light guide.
Fiber optic light guides are used in many diverse engineering and medical application areas (generally industrial engineering, lighting, traffic engineering, automobile industry, architecture, endoscopy, dental medicine). Their most important function is the transmission of as much light as possible from one location A to another location B, over short to medium distances (somewhat less than the maximum of 100 m). Frequently the light originating from a powerful light source, for example the light from a halogen or discharge lamp is coupled into the fiber optical bundle by means of optical auxiliary components, such as lenses and reflectors.
The higher the aperture (NA) of the individual fibers contained in the bundle, the greater the amount of light that can be transmitted.
The amount of light transmitted through the fiber optic light guide also depends on the transmission properties of the glass through which it is transmitted as well as the aperture. Only a core glass member of certain definite glass composition and made with raw materials of very high purity, from which it is made, guides light with as little attenuation as possible over the entire length of the light guide.
The raw materials for melting this sort of core glass are very expensive because of the required high purity, which can lead to considerable manufacturing cost for these fibers and/or the light guides made from them.
Besides the amount of light, which a fiber-optic light guide transmits, transmission of the light without chromatic effects frequently plays an important role. More or less great color shifts in the light from the light source coupled to the light guide occur because of the spectral transmission dependence of the core glass. These color shifts can cause a noticeable yellow tint to the light exiting from the light guide. Above all this is troublesome in fields, which require color neutral image reproduction, such as in the medical endoscopy with photographic image documentation for differentiation of e.g. malignant from healthy tissue.
The making of optically stepped fiber from multicomponent glass takes place either by the so-called double crucible or rod-tube process. In both cases the core and cladding glass is heated to temperatures, which correspond to a viscosity range between 104 to 103 dPas and is drawn to a fiber. So that a stable fiber with low attenuation can be made, core and cladding glass must be compatible with each other in a series of properties, such as viscosity behavior, thermal expansion, crystallization inclination or tendency. Especially contact reactions and/or crystallization may not occur at the boundary surface between the glass fiber core and cladding members. These contact reactions and crystallization may interfere with the total reflection of the light guided through it and thus may make the fiber unsuitable for the applications requiring light guidance with low attenuation. Moreover the mechanical strength of the optical fiber may be negatively impaired by crystallization.
At least three different fiber systems are known for such applications in the current state of the art.
The best-known and most widely used fiber system comprises a core glass member with a high lead composition (mostly ≧35% PbO) and an alkaliborosilicate glass acting as cladding glass. Its advantage is in the high numerical aperture achieved (up to more than 0.7 with a lead content of the core glass of greater than 50%) with reduced manufacturing costs and very good drawing properties for drawing to fiber without crystallization problems.
In contrast to that it has several disadvantages including average to poor attenuation (≧200 to 300 dB/km) and comparatively poor color shifting, chiefly caused by Pb-self-absorption (blue edge of the visible spectrum) as well as introduced impurities of strongly colored elements, such as chromium and nickel. Furthermore lead is rejected more and more as environmentally unfriendly and loading the environment. Thus in specific application areas either this fiber system is used with limitations or not at all.
A second known fiber system comprises an alkaline borosilicate glass, which is used for both the core glass member and the cladding glass member.
Different systems of this second type are described in the patent literature, e.g. EP 0018110 or EP 0081928, both from the British Patent Office. Glass compositions for optical fibers are also described in the Japanese Patent Literature, e.g. DE 29 40 451 C2 or U.S. Pat. No. 4,264,131, owned by Tokyo Shibaura Denki Kabushiki Kaisha. This glass contains a high content of alkali earth oxides and/or zirconium and germanium oxides, in order to attain the desired high index of refraction.
Its advantages include an extraordinarily lower attenuation (currently at ≦10 dB/km) and very lower color shift with currently mostly environmentally friendly raw materials (with the exception of an embodiment, which contains a large amount of barium, e.g. DE 2940451 C2). However these glasses provide glass fibers with a generally small aperture (NA) and a not very high chemical resistance. This latter disadvantage requires that the fibers are provided with a plastic jacket immediately during their manufacture after the drawing step, e.g. from the nozzle of a double crucible, as a protection against possible chemical reaction and/or mechanical action. Furthermore the very low attenuation is obtained by using more highly pure and thus even more expensive raw materials.
These latter aspects, i.e. high manufacturing cost and the plastic jacket, make the use of this fiber system as a fiber bundle for wide applications practically impossible. Furthermore this fiber system is used as individual fiber for data or energy transmission (laser fiber) in many special applications.
The third fiber system is mainly used for optical fiber bundles for light transmission. Fibers made from pure quartz provide the basis for this third fiber system.
Its advantages include an extremely low attenuation (up to ≦6 dB/km), very good color neutrality and it is sufficiently environmentally friendly. However these advantages are accompanied by a very high cost as its most serious disadvantage. Pure quartz material is extremely expensive because of its high processing temperatures. An expensive dosing process, the so-called preform, is required, in which the required index of refraction reduction of the pure quartz is obtained by introducing fluorine into the surface, which is necessary for optical isolation for light transmission in the finished fiber. Also the achievable aperture (NA) of the quartz fiber is very limited (≦0.22).
U.S. Pat. No. 4,573,762 and JP 54-087236 A discloses a stepped fiber with a core glass member and a cladding glass member. In the first named reference the numerical aperture of the glass fibers is always less than 0.5.